PENKO....you raise a very good point...however I guess it all depends on the demographics and location of the airline you work for. If you worked for a European based airline, then chances are you are better off learning something like Spanish or French....however, if your airline was say based in Australia...then it would be beneficial to learn something like Mandarin or Japanese....at the end of the day, it all depends on demographics and market location.
The most widely spoken language in the world is Mandarin....
List of languages by number of native speakers - Wikipedia, the free encyclopedia
Though I guess it would be easier for those coming from Latin-based languages to pick up Spanish and French.